exportfs: remove kernel-doc warnings in exportfs

Remove kernel-doc warning in exportfs:

fs/exportfs/expfs.c:395: warning: Function parameter or member 'parent'
not described in 'exportfs_encode_inode_fh'

Signed-off-by: Zhu Wang <wangzhu9@huawei.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>